Elasmobranchs represent a significant problem in the commercial fishing industry. Elasmobranchs are often inadvertently caught on fishing hooks and tackle directed at other more commercially valuable kinds of fish. This inadvertent catching of elasmobranchs (or other non-valued fish) is called “by-catch.” As many as 100 million elasmobranchs are killed each year as by-catch. This loss of life has resulted in a real threat to several shark species. Currently, as many as 80 species of shark are considered threatened with extinction.
Further, when elasmobranchs are caught as by-catch, fishing operations receive no return on their investment since the shark is caught on a hook that might have otherwise brought in a marketable fish. Additionally, the fishing tackle on which a shark is caught often must be cut loose for the safety of those working on the fishing vessel causing a loss of both equipment and time.
Longlining is a commercial fishing method that suffers significant losses from shark by-catch. Longlining uses multiple baited individual fish hooks with leaders strung at intervals along an often very long (2-3 miles) main fishing line. Longline fishing operations routinely target swordfish and tuna. The longline hooks, however, are not selective and elasmobranchs are sometimes caught in greater numbers than the intended catch. The result is great loss of life in elasmobranchs and significant financial losses in the longline industry. Elasmobranchs cause additional losses in the longline fishing industry by scavenging marketable fish caught on longlines before the fish may be retrieved for processing.
Elasmobranchs also represent a problem in the commercial trawling industry. Trawling is a commercial fishing method that catches fish in nets. Elasmobranchs cause significant losses for trawlers because they scavenge fish caught in trawl nets before they are retrieved for processing. As such, valuable fish are often lost to shark predation. Also, sharks often tear holes in the nets, resulting in partial or complete loss of catch and significant repair costs.
There has been a long-felt need for methods and devices to deter elasmobranchs from commercial fishing lines and nets. Attempts in the middle of the twentieth century were made to protect trawl nets with electric discharge devices. (Nelson, “Shark Attack and Repellency Research: An Overview,” Shark Repellents from the Sea ed. Bernhard Zahuranec (1983) at p. 20). Nevertheless, no commercially effective repellent has yet to be made available for reducing shark by-catch in the commercial fishing industry or for reducing loss of valuable fish or fishing tackle to shark predation. Further, Applicant is unaware of any consideration in the art of the use of magnets to repel elasmobranchs to limit by-catch and other losses from elasmobranchs.
U.S. Pat. No. 4,667,431 discloses an electric prod for repelling fish. Within the electric prod, the switch for providing electric current to the prod is a reed switch, which contains a magnet. However, the magnet is not a part of the repelling portion of the electric prod.
An effective shark repellent would not only be valuable to the fishing industry but also would be valuable for protecting humans from shark attacks. No effective repellent has yet to be marketed for limiting the risk of shark attacks faced by humans exposed to elasmobranchs. Over the last 50 years antishark measures employed to protect humans from shark have included electrical repellent devices (Gilbert & Springer 1963, Gilbert & Gilbert 1973), acoustical playbacks (Myrberg et al. 1978, Klimley & Myrberg 1979), visual devices (Doak 1974) and chemical repellents (Tuve 1963, Clark 1974, Gruber & Zlotkin 1982). None of these procedures proved satisfactory in preventing shark attacks. (Sisneros (2001)). As such, the long felt need for an effective repellent has not been satisfied.
Researchers have historically used several bio-assays to determine if a repellent evokes a flight response in shark. One such bio-assay measures the effect of a repellent on a shark that is immobilized in “tonic immobility.” Tonic immobility is a state of paralysis that typically occurs when a shark is subject to inversion of its body along the longitudinal axis. This state is called “tonic,” and the shark can remain in this state for up to 15 minutes thereby allowing researchers to observe effects of repellents. After behavioral controls are established, an object or substance that has a repelling effect will awaken a shark from a tonic state. Researches can quantify the strength of a repellent effect from these studies.
Another bio-assay employs a Y-shaped maze wherein a shark is exposed to a choice between two paths containing the same olfactory stimulus. One path exits the maze without a repellent while the other contains a repellent. If the sharks consistently choose the path without the repellent or consistently become agitated in the path having the repellent, researchers may conclude the repellent is effective.